1. Field of the Invention
The present invention relates to multi-head thermal printers and, in particular, to thermal printers in which multiple print heads are used to print a single image in the form of multiple joined segments.
2. Related Art
Various kinds of printers are well-known in the computing and digital image arts. Such printers include, for example dot-matrix printers, laser printers, inkjet printers and thermal printers. The focus of the present discussion is on thermal printers, so-named because they use thermal energy (heat) to produce printed output. More specifically, thermal printers typically contain a linear array of heating elements (also referred to herein as “print head elements”) that print on an output medium by, for example, transferring pigment from a donor sheet to the output medium or by initiating a color-forming reaction in the output medium. The output medium is typically a porous receiver receptive to the transferred pigment, or a paper coated with the color-forming chemistry. Each of the print head elements, when activated, forms color on the medium passing underneath the print head element, creating a spot having a particular density. Regions with larger or denser spots are perceived as darker than regions with smaller or less dense spots. Digital images are rendered as two-dimensional arrays of very small and closely-spaced spots.
A thermal print head element is activated by providing it with energy. Providing energy to the print head element increases the temperature of the print head element, causing either the transfer of pigment to the output medium or the formation of color in the receiver. The density of the output produced by the print head element in this manner is a function of the amount of energy provided to the print head element. The amount of energy provided to the print head element may be varied by, for example, varying the amount of power to the print head element within a particular time interval or by providing power to the print head element for a longer time interval.
A single thermal printer may include multiple thermal print heads, which may, for example, be staggered with respect to each other. One example of this kind of printer is described in U.S. Pat. No. 4,660,052 to Kaiya et al., and is described as a heat sensitive recording apparatus with multiple thermal heads disposed in a staggered arrangement along two platen rollers. The apparatus has alternate image segments printed on a first platen roller by a first set of print heads. The intervening segments are filled in by a second set of print heads printing on a second platen roller. The heads are arranged such that the printing of the second set of print heads overlaps the printing of the first set of print heads, forming “stitching” regions between each pair of adjacent segments in which the printing may be adjusted to obscure the presence of a transition from one to the other. In this patent, the method of journey is described as a simple abutment in which a point of transition is chosen near the center of each stitching region. All pixels to the left of the transition are printed by the left-hand print head of the pair of overlapping heads, and all pixels to the right of the transition points are printed by the right-hand print head of the pair. This method of journey is troublesome, because it lacks robustness toward imperfections in the printer hardware. For example, if the paper motion is not perfectly perpendicular to the print heads, then the paper may shift slightly to the right or left when traveling from one set of print heads to the other, thereby opening a gap in the stitch or causing an overlap of image segments. In addition to these mechanical imperfections, the thermal print head heats up as it prints, and thermal expansion of the heads can cause a visible overlap of image segments.
U.S. Pat. No. 4,997,410 to Onuki and Denda describes specific means for implementing an abutted joint as described above by means that distribute stitching-region data to the appropriate print heads, depending on whether they are to the right or left of a chosen transition point. This patent describes means for manual readjustment of the stitch so as to eliminate any visible gap or overlap, and also describes means for automatically compensating for the effects of thermal expansion of the heads. It would be preferable that no such manual adjustments were required for proper operation.
U.S. Pat. No. 5,119,108 to Hatakeyama describes a very similar system, but adds the recommendation that the image segments be overlapped by 2-4 pixels, thereby eliminating (for all practical purposes) the possibility of a gap opening up between the image segments. This, of course introduces a 2-4 pixel wide region of higher printed density, which the inventors apparently consider to to be unobjectionable due to the very narrow width of the overlap. This imperfection, however, extends the full length of the image, and may be visible despite its narrow width.
A solution to this problem is proposed in U.S. Pat. No. 5,450,099 to Stephenson and Fiscella. This patent describes a stitch that is more sophisticated than the simple abutted joint. On each line in the stitching region, the pixels to be printed are divided in a random pattern between the two print heads. Each print head prints approximately one-half of the pixels in the stitch, interleaved so that each pixel is printed either by one or by the other of the two print heads. On each line, the random division of pixels is changed so that there is no recurring pattern from line to line. This avoids correlated defects that extend the full length of the image, but it does place demands on the mechanical and thermal tolerances of the printer, as a misregistration of the patterns will result in significant uncontrolled changes in the printed density of the stitch region. In the case of misregistration, approximately 25% of the pixels will be printed by both print heads, and 25% of the pixels will not be printed by either print head. These randomly occurring increases and decreases of density do not compensate for each other, and an imperfect density is printed.
In view of the drawbacks of these prior-art methods of stitching image segments in thermal printers, there is a need for a method of joining image segments such that mechanical imperfections in the printer hardware, and thermal expansion of the printer components, will not result in visible artifacts in the printed image. The consequence of such a method would be an improvement of image quality, and a reduction in the cost of wide-format thermal printers (since a high-precision transport mechanism would not be required).